I'm not sure what your point is here. Yes, scaling typically involves adding hardware as well as tuning your code. What does this have to do with physical tiers vs. logical tiers?

Just reving up :)

First, you only separate things if they are so uneven in terms of the resources they require that normal load-balancing (treating all requests for dynamic pages equally) doesn't work. This is very rare. Second, you NEVER change links! There is no reason to change your outside URLs just because the internal ones changed. If you want to change where you route the requests internally, use your load-balancer or mod_rewrite/mod_proxy to do it.

Sometimes, you don't know ahead of time what gets used more than naught. Usually products evolve. And using mod_proxy or the likes is a cludge. What happens when you move something more than once? Regardless, doing the extra work for moving stuff around is ugly anyway. It's unnecessary work if you seperate your "hard work" away.

How does this help anything? The amount of work is still the same, except you have now added some extra RPC overhead.

Yeah, but you've moved your hard work away to pools dedicated to resources that can handle it. Things like static stuff doesn't need to hit db resources ever. Things that need to do intensive stuff goes to a pool that does that. It breaks down more and more.

Can you explain what you're talking about here? Are you saying that some of your requests will not actually need to use the db layer or application layer?

Ok, for instance, let's say your login page is realy fast, and so is your page after auth. Now let's say your preferences page is REALLY slow. It takes up lots of resources since it gets pegged a lot. Seperating out the logic that is so slow because it gets hit so much can be moved to it's own pool. Now you have one set { login, homepage} and another {preferences} which can be in two different pools. The poools don't have to be server farms of machines, but when things get more complex or not, you can allocate or take away resources for them.

Running well is a relative term. They will not run anywhere near as fast as they would without the RPC overhead. I'm not making this stuff up; this is coming from real applications in multiple languages that I have profiled over the years. Socket communication is fast, but it's much slower than most of the other things we do in a program that don't require inter-process communication.

You are 100% right, but the time difference is insignificant. A pooled connection vs an in-machine IPC call's speed is a magnatitude faster, but in terms of user experience, it is so small, that you can hardly notice.

And that's the other problem: the RPC overhead forces you to replace a clean OO interface of fine-grained methods with "one fell-swoop..." This is one of Fowler's biggest complaints about distrubuted designs.

No it doesn't. They are called transfer objects. Just a basket where you say, I want NN and it returns back in one request. Nothing particularly messy about it. If you do it generic enough, I'm sure that you could abstract it out to many many uses and not 1 dedicated object transfer.

I'm not sure where you're getting this from. mod_rewrite (the most common choice for doing things based on URL) is very fast, and the hardware load-balancers are very very fast.

Yup, but then again, so are RPC calls :)

What's so bad about a web farm? Every large site requires multiple web servers. And which parts are costly?
I think you are imagining that it would take less hardware to achieve the same level of service if you could restrict what is running on each box so that some only run "business objects" while others run templating and form parsing code. I don't see any evidence to support this idea. If the load is distributed pretty evenly among the machines, I would expect the same amount of throughput, except that with the separate physical tiers you are adding additional load in the form of RPC overhead.

Ah.. that's the thing. evenly. You don't want everything running evenly. If slashdot could seperate out say, it's front page logic from its comment logic, then the front page will always be speedy and the comments section be its relative speed. As more people do commenty stuff, the home page stays right quick.

Think of it like this: you have a request that takes 10 resource units to handle -- 2 for the display and 8 for the business logic. You have 2 servers that can each do 500 resource units per second for a total of 1000. If you don't split the display and business logic across multiple servers and you distribute the load evenly across them, you will be able to handle 100 requests per second on these servers. If you split things up so that one server handles display and the other handles business logic, you will max out the business logic one at 62 requests per second (496 units on that one box). So you buy another server to add to your business logic pool and now you can handle 125 requests per second, but your display logic box is only half utilized, and if you had left these all together and split them evenly across three boxes you could have been handling 150 at this point. And this doesn't even take the RPC overhead into account!

Distributed objects sound really cool, and there is a place for remote cross-language protocols like SOAP and XML-RPC, but the scalability argument is a red herring kept alive by vendors.

Or not. Say the cost of rendering a page is small, s. You have 1 server that can deal with 10 connectiosn really well. On that same server, you have b, a big process that takes a lot of time. and 10 tiny processes t. b bogs down t to the point of "slow". You add another server. Things get "better" but imagine if you tier'ed it. You have three machines. One that handles s, one that handles b and one that handles t. the t-machine will alwyas run fast. And as more people use b, you add more resources for b. But as b continuously gets more and more poeple, T NEVER slows down. THAT is what you want to avoid.

You don't want to add to the entire pool and have to speed up everything in one fell-swoop. It's the same reason you have a 3d video card and a cpu completely seperate. Totally seperate purposes for different things. If your cpu gets pegged for whatever reason, your 3d video doesn't. You can tweak the 3d card or even replace it w/o having to go through hell.

Btw, there's always statistics, but we can always skew them in various ways. I can quote numbers any way i want, even to refute my own argument. But you can't refute that if T stays simple and fast, and B gets more complex, that T would be unaffected. :)

Btw, my GF is ticked from all the typing you are making me doing. She was on the phone and kept thinking i was doing something more important, from the clakity clak i was making :)